Car wash would be a bad idea (salt, grit, etc.), of course. But I suspect you meant to say you are NOT implying that.
Even with clean water, corrosion is an issue with pressure washing. Raising the pH might help some (but keep it away from electronics). But you are going to force water into all manner of places it could linger and cause trouble.
High pressure washing could be abrasive with existing grit, though probably not as bad as leaving it there.
If your machine has moglice surfaces, the moglice may absorb emulsified water and expand causing the ways to seize:
http://www.practicalmachinist.com/vb/showpost.php?p=1117975&
postcount=10
I am also reminded of the time a friend cleaned some oil off the bottom of car by tossing some laundry or dish detergent up till it stuck and hosing off with a garden hose. Rusted very quickly. If you are removing oil, you need to replace it quickly; if you are removing oil in places you aren't aware of or can't reach, you could have a problem.
You run the risk of degreasing bearings, ball nuts, forcing crud into the encoder scales, etc. At 1500PSI, you might not just get dirt in the encoder scales, you might damage them.
A CNC machine is likely to be designed to deal with a fair amount of spray but with a pressure washer the spray is likely to be coming from angles that would not occur in normal use and doing so far more vigorously. Parts that can deal with spray raining down from above would be subjected to spray coming upward.
I have never tried cleaning a machine in your condition, so take this with a grain of salt. If I was going to resort to draconian pressure washing, I would probably try a light oil or a soluble oil cutting fluid. Most of that crud was probably dissolved in oil or soluble oil at one time. Might even use the machines coolant pump with a hose and a nozzle made from a piece of tubing crimped around a thin piece of feeler gage (then removed) or other somewhat aggressive spray nozzle (and even these may be too aggressive) and just slowly guide it along the surfaces to be cleaned with wetting and soaking time as well. And filter anything that returned to the sump before re-using. And mask off areas where over-spray could do damage and try to catch as much overspray as I could (maybe direct the overspray into a shop vac). You shouldn't need absurdly high pressure. And I would still restrict that to areas where I could contain the side effects. One place I worked put parts in a dishwasher to degrease (these came from hostile environments including coal mines). A dishwasher doesn't use 1500PSI, it just sprays continuously for about an hour so you have a combination of spray and soak time. High volume, low pressure. If I thought the machine could handle it, might chuck a dishwasher spray assembly in the spindle; the result would be somewhat compatible with the direction and pressure of coolant spray in normal operation. I.e. outward from the spindle and down. You could get about the same results by chucking various different shaped tools (including a slitting saw or disk for a high spray onto the walls) in the spindle with coolant to get different spray patterns. Or use a disk/slitting saw and a variety of spindle speeds. But I might be inclined to disassemble and only subject individual parts to draconian treatment if I was going to get drastic. If the surfaces you can see need cleaning, the ones you can't see probably do as well. And there would be less collateral damage. Disassembly isn't without its hazards, either. But if the machine has been neglected, how much of that crud has worked its way into the areas you can't see? This section deals with some possible precautions for making a foolish activity (pressure washing) slightly less foolish.
You did say low pressure. I would define low pressure as anything that was as low as the original coolant system pressure, or lower. But angle still needs to be considered. Same pressure directed in ways that do not occur during normal machine operation may do more harm than good.
I would also ask if this is a, if it isn't broke, don't fix it situation? Be a shame to convert the machine into two tons of scrap metal trying to fix a problem that was really minor to start with. Is the grime in places where it will cause trouble or is it just the cosmetic effect of the coolant evaporating overnight and depositing suspended/dissolved grime from poorly maintained coolant on the walls and floor of the enclosure and other non-critical surfaces? How much of that might just go away on its own with normal coolant use (well filtered) and wiping the surfaces dry at the end of each day (to prevent further deposits and do some cleaning), replacing coolant frequently at the beginning, and following the directions for machine cleaning when replacing coolant. Of course, you probably have biofilms which you would do well to get rid of (as they will cause bacterial growth to reemerge after a coolant change) - and they might account for some of the appearance. But giving the machine a fresh coolant change and letting it do some simulated running (without actually cutting anything) for 8 hours with an hourly wipe down or scrub, might loosen up a lot of crud. If you really think the ways are up to operating without disassembly, then include some random table movements or sweep patterns so the spray reaches other areas. A soluble oil cutting fluid bears some resemblance to soapy water. This would create conditions somewhat resembling the inside of a dishwasher (including high humidity). Then change out the coolant (don't omit the disinfecting stage). Do it again the next day. Then maybe use the machine normally while being scrupulous about cutting fluid maintenance and end of day wipe down. Since you are exploiting normal operating conditions for cleaning, you shouldn't damage the machine like you might by trying a manual low pressure wash as long as the spindle bearings and ways are free of grit and properly lubricated. I figure the machine is probably designed to deal with any amount of spray within normal coolant system pressure or centrifugal force propelled spray as long as it originates from the bottom of the spindle and is directed horizontal or lower and consists of approved cutting fluids. Violate any of those conditions, and you may be asking for serious trouble. Too much pressure or from the wrong direction and you could violate the seals or physical barriers that depend on gravity or point of origin. It is like the difference between rain falling on your roof and being driven against the walls by normal winds and standing on the ground spraying a garden house up through the eave vents and up through the joints in the siding. That is part of why you aren't supposed to use compressed air or pressure washers to clean the machine.
Here are a couple online books on cutting fluid maintenance:
http://www.iwrc.org/downloads/pdf/cuttingFluid03.pdf
http://www.p2pays.org/ref/06/05828.pdf
I have assumed here that the grime was deposited during operation and any operations wouldn't do more damage than the last few days of operation of the machine did. Some of it may have been deposited after the machine was taken out of service. I.e. dust, etc. embedding itself in oil residue during storage.
dkmc didn't mention if purple power had actually been tested on machine tools. The website is rather devoid of useful information. The MSDS says the concentrate has a pH of 10-11 which would presumably inhibit rust (actually, it is too high and might cause corrosion) but diluted the pH might be too low. But letting things soak and then cleaning before it dries sounds good to me. I am just suspicious of undocumented chemicals, especially those that weren't designed specifically for a particular application.
This is outside my experience and I haven't even seen pictures of your machines condition, so it is up to you to decide what, if any, of this is appropriate to your situation.